1. Field of the Invention
The present invention relates to a radiation image pickup apparatus and a method of driving the same, and more specifically to a radiation image pickup apparatus (a radiation beam reader) reading information based on radiation by converting the wavelength of radiation represented by α-rays, β-rays, γ-rays or X-rays.
2. Description of the Related Art
In a radiation image pickup apparatus, a charge based on input information photoelectric-converted by a photoelectric conversion section is transferred to an external capacity, and the charge is converted into a signal voltage in the external capacity. When a charge is transferred from the capacity of the photoelectric conversion section to the external capacity to be converted into a signal voltage, a relatively large S/N is achievable.
When a configuration in which a plurality of pixels are arranged is used, the length of a signal line reading signals from pixels is increased according to the number of pixels; therefore, a parasitic capacitance may be formed. For example, the case where 2000 horizontal×2000 vertical pixels with a size of 200 μm×200 μm are arranged to form an area sensor with a size equivalent to an X-ray film, for example, 40 cm×40 cm will be considered below.
In the case of the area sensor with a size equivalent to the X-ray film, a capacity is formed by an overlap between a gate electrode and a source region of a transistor transferring a charge. The overlap depends on the number of pixels; therefore, when an overlap capacity Cgs has approximately 0.05 pF at each pixel, a capacity of 0.05 pF×2000 pixels=100 pF is formed in one signal line.
As the capacity (sensor capacity) Cs of the photoelectric conversion section is approximately 1 pF, an output voltage V0 of the signal line is V0={Cs/(Cs+Cgs×1000)}×V1 where a signal voltage generated in a pixel is V1, and the output voltage is approximately 1/100. In other words, in the case where an area sensor with a large area is configured, the output voltage largely declines.
Moreover, to read moving pictures under such circumstances, sensitivity and a high-speed operation allowing the photoelectric conversion section to read 30 or more pictures per second are necessary. In particular, there is a demand for minimizing the dose of X-rays to be applied in nondestructive testing including X-ray diagnosis in medical care; therefore, higher sensitivity allowing a signal charge amount to be increased by 100 to 400 times is desired.
On the other hand, in related art, as described in Japanese Unexamined Patent Application Publication No. 11-307756, specifically in paragraphs 0040 to 0044 and FIG. 7 therein, a configuration in which a field-effect transistor having a gate receiving a signal charge generated in a photoelectric conversion section is included and a source follower circuit reading out a signal voltage based on the signal charge to a signal line by the field-effect transistor is arranged in each pixel is used. In the source follower circuit, high-speed signal readout is allowed even in the case where a capacity formed in the signal line is large.
In a radiation image pickup apparatus including a source follower circuit of this kind, each pixel includes a drive element section including a bottom-gate field-effect transistor (pixel transistor) and a PIN (Positive Intrinsic Negative Diode) photodiode. The field-effect transistor includes a semiconductor layer (channel layer) made of microcrystalline silicon or polycrystalline silicon.